1. Field of the Invention
The present invention relates to systems and methods receiving broadcast signals, and in particular to a system and method for receiving satellite broadcasts while reducing interference from terrestrial sources or from satellite sources such as nongeostationary fixed satellite service networks.
2. Description of the Related Art
It has been proposed to cooperatively share the current Broadcasting-Satellite Service (BSS) frequency bands to allow additional programming material to be transmitted to BSS users or subscribers using the same frequency bands as currently used by BSS satellites. This may be implemented through the use of non-geostationary orbit (GSO) and/or terrestrially-based transmitters to transmit the additional programming. Such systems typically rely on spatial diversity to minimize the probability of interference. This usually requires a BSS satellite ground antenna having highly directional, monocular sensitivity characteristics in order to realize low interference levels.
Unfortunately, existing BSS antennae do not exhibit a highly directional sensitivity characteristic. Instead, as described in application Ser. No. 09/480,089, entitled xe2x80x9cMETHOD AND APPARATUS FOR MITIGATING INTERFERENCE FROM TERRESTRIAL BROADCASTS SHARING THE SAME CHANNEL WITH SATELLITE BROADCASTS USING AN ANTENNA WITH POSTERIOR SIDELOBES,xe2x80x9d which application is hereby incorporated by reference, existing BSS antennae exhibit a sensitivity characteristic that includes substantial sensitivity in a rearward direction. They also exhibit a sensitivity characteristics in the sideward and upward directions. This sensitivity can result in substantial interference between transmissions from BSS satellites and transmissions from non-GSO or terrestrial sources.
U.S. Pat. No. 3,430,244, issued to H. E. Barlett et al. discloses a transmitting reflector antenna. The transmitting antenna includes a solid dielectric guiding structure imposed between the feed and the reflector. The dielectric surface acts as a lens to direct the radiation emanating from the feed at the reflector surface. Because the incident angle of the electromagnetic energy from the phase center of the horn to the lens is at a small angle, the electromagnetic energy is largely reflected. If not for the lens, the electromagnetic energy would emanate from the phase center of the horn and continue beyond and behind the reflector surface, thus creating spillover. While this design reduces spillover, this design requires use of an expensive dielectric structure extending from the horn to the reflector surface, thus complicating installation, and requires a modified reflector surface in order to direct the rays where required. The design can also result in significant phase distortion.
U.S. Pat. No. 3,176,301 issued to R. S. Wellons et al. discloses an antenna design having multiple feeds. A cylindrical metallic shield is placed on the periphery of the reflector and a second cylindrical metallic shield is placed surrounding the feeds to reduce spillover. While this design can reduce spillover, the metallic surface permits reflections within the shield itself, potentially compromising the spillover reduction, and permitting distortion of the received signal. The reflections within the metallic shield are also made worse because the shield itself is distant from each of the horns. Further, the metallic shield is not easily attached to the assembly of horns.
U.S. Pat. No. 3,706,999, issued to Tocquec et al. discloses a Cassegraninan antenna with a design that is said to reduce spillover energy. However, exising BSS antennae are simple offset reflector designs and cannot be easily modified in accordance with the disclosed Cassegranian design.
U.S. Pat. No. 4,263,599, issued to Bielli et al. discloses a parabolic reflector antenna having a reflector periphery lined with absorbent material to reduce spillover. While design reduces spillover, it requires the use of a substantial amount of absorbent material.
U.S. Pat. No. 4,380,014, issued to Howard, U.S. Pat. No. 4,803,495, issued to Monser et al., U.S. Pat. No. 5,905,474 issued to Nagi et al., and U.S. Pat. No. 5,959,590 issued to Sanford et al. each disclose designs which reduce spillover. However, in each case, the design disclosed is not one that can be obtained with simple modification of existing BSS antennae.
What is needed is an inexpensive, but effective way to modify the sensitivity characteristic of existing BSS antennae to reduce the interference from non-GSO and terrestrial broadcast sources. The present invention satisfies this need.
To address the requirements described above, the present invention discloses an antenna for receiving electromagnetic energy from a first transmitter and substantially rejecting electromagnetic energy from a second transmitter spatially diverse from the first transmitter. The antenna comprises a reflector having a reflecting surface for reflecting and focusing the electromagnetic energy from the first transmitter to at least one focal point; a feed assembly for receiving the reflected electromagnetic energy, the feed assembly having a sensitive axis facing the reflecting surface wherein the feed assembly and the reflector together define a spillover region bounded by a feed assembly beamwidth extending from the sensitive axis at least partially beyond the reflector surface; and an electromagnetic energy absorber, attached to the feed assembly and disposed at least partially between the spillover region and the feed assembly. The present invention is also described by a method of receiving electromagnetic energy from a first transmitter and substantially rejecting electromagnetic energy from a second transmitter spatially diverse from the first transmitter. The method comprises the steps of receiving electromagnetic energy from the first transmitter reflected by a reflector surface in a feed assembly, the feed assembly and reflective surface together defining a spillover region defined by a feed assembly beamwidth extending from a feed assembly sensitive axis at least partially beyond the reflector surface; and absorbing the electromagnetic energy from the second transmitter with an absorber coupled to the feed assembly and disposed at least partially between the spillover region and the feed assembly.